Make repr-reliant test less specific

IPython/utils/tests/test_jsonutil.py

@@ -58,7 +58,8 b' def test():'
 
 def test_lambda():
     jc = json_clean(lambda : 1)
-    nt.assert_true(jc.startswith('<function <lambda> at '))
+    assert isinstance(jc, str)
+    assert '<lambda>' in jc
     json.dumps(jc)